1. Technical Field
This invention relates generally to wind turbines and the like, and more particular to a vertical axis wind turbine that utilizes the kinetic energy of moving air to provide rotational energy useable for generating electric power and/or other useful purposes.
2. Description of Related Art
Wind turbines usually take the form of horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). By virtue of their vertical axes of rotation, VAWTs do not require alignment with the windstream. In addition, drive train components can be located at ground level instead of being mounted higher above ground at HAWT rotor level. For those and other reasons, VAWTs attract attention . . . especially for commercial electric power generating purposes.
VAWTs include drag-based designs and lift-based designs. U.S. Pat. No. 226,357 issued Apr. 6, 1880 to Saccone, for example, describes an early, drag-based, vertical axis, windmill design. Designed over twenty years before the Wright brothers"" flight, the windmill uses flat xe2x80x9cfansxe2x80x9d mounted pivotally on a support structure in order to catch the wind and cause the support structure to rotate. As the fans orbit the vertical axis, they pivot between a downwind orientation, in which each presents a broad profile in order to catch the wind, and an upwind orientation in which each presents a narrower profile for less drag. The windmill was designed without the benefit of aerodynamic design and performance theory. Operation is jerky, rough, and slow as the fans are continually pulled out of position by centrifugal force. The fans provide drive only intermittently during a somewhat small portion of each rotation. Upright bars at the outermost ends of the fans are highly disruptive to airflow. The system cannot achieve rotor speeds faster than wind speed.
U.S. Pat. No. 2,038,467 issued some fifty-six years later on Apr. 21, 1936 to Zonoski describes another vertical axis, drag-based, windmill design utilizing flat xe2x80x9cvanesxe2x80x9d on a rotatable frame. The two-phase vanes are better balanced. As they orbit the vertical axis, they pivot about 170 degrees, or so, between a high-drag downwind orientation and a low-drag upwind orientation. Although the windmill shows potential for drag rotation over 180 degrees of each revolution, wind shadow and vane interference reduces overall effectiveness, and relative wind reduces the draft phase to less than 180 degrees. U.S. Pat. Nos. 4,408,956; 4,474,529; and U.S. Pat. No. Des. 300,932 show other drag-based designs.
U.S. Pat. No. 4,383,801 issued May 17, 1983 to Pryor shows a lift-based VAWT. It includes vertically aligned airfoils mounted pivotally on a rotatable support. As the airfoils drive the support, they orbit the vertical axis. Meanwhile, a wind-vane-controlled pitch adjustment continually orients the airfoils relative to the wind direction. The machine detects wind direction by means of a vane and positions the controlling pitch flange accordingly. The mechanism is somewhat complicated, and positioning of the airfoils (angle of attack) is optimized only in the directly upwind and directly leeward positions, using crosswind lift force in both cases. In addition, FIGS. 8 through 11 in the patent illustrate somewhat complex mechanisms for manipulating the airfoil shapes. The airfoils are underutilized during most of each rotation. The additional control appears to be an attempt to improve the efficiency of the machine. In a class of VAWTs called cycloturbines, the pitch of the airfoils is controlled to create crosswind lift, but they must run at rotor speeds in excess of wind speed to be effective. They also frequently have difficulty self-starting.
Thus, the prior art has progressed to the use of airfoils in lift-based VAWT designs. More efficient conversion of wind energy is still desirable, however, along with better VAWT mechanical attributes. So a need exists for a better VAWT . . . preferably a lift-based VAWT incorporating benefits of modern aerodynamic design and performance theory.
It is an object of this invention to overcome the forgoing and other disadvantages of prior art wind turbines. This object is achieved by providing a VAWT (referred to herein as a vertical axis wind engine) having xe2x80x9cfree flyingxe2x80x9d airfoils that self position according to the local dynamic conditions to which they are subjected, thus creating a condition of equilibrium under which a highly efficient means of wind energy extraction may be established. More particularly, the vertical axis wind engine includes a rotor mounted on a support structure for rotation about a vertical axis. At least one airfoil mounted pivotally on the rotor (preferably more than one) causes the rotor to rotate under influence of the wind. The airfoil is mounted on the rotor so that it is free to pivot between preset first and second limits of pivotal movement (e.g., set by stop mechanisms). That arrangement enables the airfoil to align according to the wind as it orbits the vertical axis, thereby achieving better conversion of wind energy to useable rotational energy by combining lift and drag characteristics at low speeds and shifting to lift-only characteristics at rotor speeds approaching or exceeding local wind speed.
The dynamic phase lag effect of the free flying airfoils creates a xe2x80x9cvirtual stop.xe2x80x9d Recall the Law of Conservation of Angular Momentum and consider its influence in combination with airfoil responsiveness to the instantaneous force of the true relative wind (TRW) acting on an airfoil. The result is that the airfoil resists rotational changes along its pivotal axis and shifts out of phase relative to its rotor position. That is what the stops do also. The stops cannot be completely eliminated, however, because they are required during start-up, operation at low speed, heavy load conditions, turbulence, and wind direction shifts requiring reorientation and stabilization of the system. The stops are important in getting the system up to equilibrium speed.
To paraphrase some of the more precise language appearing in the claims and introduce the nomenclature used, a vertical axis wind engine constructed according to the invention includes a support structure, a rotor mounted rotatably on the support structure for rotation about a vertical axis, and at least one airfoil mounted on the rotor for causing the rotor to rotate about the vertical axis in response to wind passing the wind engine. The airfoil has vertically extending leading and trailing edges, an angle-of-attack axis extending horizontally through the leading and trailing edges, and a pivotal axis extending vertically intermediate the leading and trailing edges. The airfoil is mounted on the rotor for pivotal movement about the pivotal axis and the rotor includes means for limiting pivotal movement of the airfoil to first and second limits of pivotal movement.
According to a major aspect of the invention, the airfoil is mounted on the rotor so that the airfoil is free to pivot about the pivotal axis intermediate the first and second limits of pivotal movement as the rotor rotates about the vertical axis. That arrangement enables the airfoil to align the angle-of-attack axis continually according to the wind as it orbits the vertical axis. Preferably, the wind engine has more than one airfoil and the rotor includes first and second stops for each airfoil that limit pivotal movement to a radially aligned first limit and a tangentially aligned second limit. According to another aspect of the invention, multiple wind engines are stacked. Yet another aspect provides an exponentially shaped structure surrounding the vertical axis that funnels wind toward the rotor.
In terms of its many advantageous design features, the wind engine of the invention is a vertical axis wind engine with one or more self-positioning airfoils that achieve better conversion of wind energy to useable rotational energy by optimizing the lift and drag characteristics within the appropriate rotor speed ranges. The design uses no cams, gears, levers, or other mechanisms to position the airfoils, thereby reducing design complexity, minimizing frictional overhead, and increasing working efficiency. The energy transfer cycle is optimized by the application of aerodynamics based on airplane and helicopter airfoil flying and stalling characteristics and the physics of conservation of rotational energy. The airfoils optimize each phase in the rotational cycle, using four distinct methods of applying motive force to the rotor armature.
Wind forces and armature-constraining action alone establish airfoil positions. Airfoils rotate freely through an arc of approximately 90 degrees, bounded by stop mechanisms. The airfoil""s span of travel is from a radial line along the mounting arm (radially aligned relative to the vertical axis) to a perpendicular position (tangentially aligned relative to the vertical axis). The design allows for each airfoil to set its own instantaneous angle and to adjust to conditions of relative wind, wind shift, and so forth occurring outside and within the wind engine, without external adjustments or mechanisms, wind vanes, centrifugal governors, or other controlling devices. Individual airfoils adjust to local conditions based on changes of rotor speed, turbulence, true relative wind, and other factors affecting each of them independently.
The wind engine is self-starting with two or more airfoils from any rotor position and/or wind condition. The wind engine may be configured as a low-rpm, high torque device for applications such as water pumping, but may also be made to adhere more closely to Betz""s Theory (1926) of maximum efficiency. Low-rpm, high torque devices are inherently inefficient because power is a product of torque and rotor speed. Higher rotational RPMs are also more favorable for driving electrical generation equipment. Wind energy is derived by reducing the velocity of an air mass. According to Betz, an ideal turbine system should lower the speed of the wind by a factor of only about one-third. It should also create the minimum amount of interference to the passing wind stream. To reduce rotor xe2x80x9csolidity,xe2x80x9d airfoils can be constructed in a narrow aspect ratio and allowed to rotate faster on a larger rotor radius. The objective is to sweep a large surface area at the highest speed possible, using a minimum number of airfoils (two or three). Despite these significant changes in operating parameters, all aspects of the invention""s fundamental working method would remain unchanged. In order to meet Betz""s criteria for efficiency, a wind turbine must be able to exploit crosswind lift to produce the necessary airfoil speeds. This is the primary reason why drag-based VAWT designs have garnered less favor in the research community than the HAWT machines.
The wind engine design is scalable, both in terms of overall size and in terms of the number of airfoils utilized. It may be configured in a smaller radius for higher RPM operation, or larger diameter for lower RPM operation and higher torque, or it may be built on a very large scale for power-grid applications.
The wind engine design allows for easy aesthetic placement in the landscape and is ideally suited for supplemental consumer energy, emergency supply, remote sites, camps, or ranch and farm use. It is suited to packaging that is modular and easily stackable, allowing it to be bolted together to create a self-contained tower structure.
The wind engine design exploits a unique combination of aerodynamic lift and drag forces, which can allow for potential top rotor speed (TSR) in excess of wind speed. As rotor speed increases above wind speed, airfoils transition into lift-only mode. If rotor speed decreases below wind speed under load, the system functions effectively due to the large wind exposure area. The airfoils are designed to combine wing/lift as well as turbine/drag functions.
The wind engine design introduces a five-step sequence to wind turbine technology. Power is produced in four of the following five phases:
1. Upwind Lift Phase. This begins approximately in the upwind position and continues to approximately 60 degrees past it, depending on wind and rotor speed conditions.
2. Downwind Drag Phase. This begins at approximately 60 degrees downwind and continues to around the 120 degree position.
3. Transitional Phase. At about the 120 degree position, the airfoil rotates its orientation by 90 degrees and converts its rotational energy into rotor thrust by the law of conservation of rotational inertia.
4. Leeward Lift Phase. Positioned crosswind by the transitional phase, the airfoil now sweeps across the leeward side of the system.
5. Upwind Phase. The airfoil returns to windward, positioning itself for minimum drag.
Thus, the wind engine of the instant invention is a new lift-based VAWT design providing significantly improved performance, mechanical attributes, and aesthetics. The following illustrative drawings and detailed description make the foregoing and other objects, features, and advantages of the invention more apparent.